The convention has implemented a valve timing control apparatus for changing opening/closing timing of an intake valve and/or an exhaust valve according to an operational condition of an internal combustion engine (“engine”). For instance, there is known a mechanism for changing the opening/closing timing of the intake valve which is opened/closed in association with rotation of a cam shaft, by changing a rotational phase of the cam shaft relative to a crank shaft. Incidentally, the intake valve and the exhaust valve each has its own timing favorable for starting the engine. And, this opening/closing timing often differs from the opening/closing timing of the same valve during traveling of the vehicle. More particularly, the rotational phase of the cam shaft at the time of start of engine is often located at an intermediate position between the angle advancing side and the angle retarding side. For mechanically fixing this position as an initial phase suitable for engine start, there is known a variable valve timing mechanism having a locking mechanism for locking the rotational phase of the cam shaft at the initial phase (see. e.g. Patent Document 1). With this variable valve timing mechanism, after the engine starts at the initial phase and subsequently enters its operational state and when the hydraulic pressure builds up thereafter, the locking mechanism is released, thus allowing phase control suitable for the operational state.
Further, in order to ensure displacement, at the time of engine start, from the retarding phase side to the intermediate locking position (initial phase) that is the relative phase suitable for engine start, a valve timing adjusting apparatus is known that has an angle advance assisting spring for assisting the phase displacement toward the advancing side (see e.g. Patent Document 2). With this valve timing adjusting apparatus, the range of urging phase of the advance assisting spring is set to the sum of the intermediate locking phase (initial phase) and 10 degrees, as measured from the maximal retarding phase. With this arrangement, even in the event of drop in the hydraulic pressure at the time of stop of the engine, the relative phase will have been displaced to the position beyond the initial phase, due to the urging force of the advance assisting spring, and at the time of start of the engine, by a cam reaction force, the relative phase will be displaced toward the retarding side, against the urging force of the advance assisting spring and the relative phase will be locked eventually at the intermediate locking position.
With the above-described valve opening/closing timing control technique according to Patent Document 2, at the time of stop of the engine, with utilization of the urging force of the advance assisting spring, the relative phase between the crank shaft and the cam shaft is displaced to a phase slightly beyond the initial phase; whereas, at the time of the start of the engine, the relative phase of the cam shaft is locked to the initial phase by means of the cam reaction force, and the force resulting from e.g. viscosity of oil, effective in the retarding direction, thus improving the start performance of the engine. Further, for allowing speedy displacement of the relative phase from the retarding side to the initial phase at the time of stopping the engine, it is desired that the urging force of the advance assisting spring be strong. However, if the force effective in the retarding direction such as the cam reaction force or due to the oil viscosity, is weak, locking to the initial phase takes long time due to the resistance of the advance assisting spring, or depending on the case, the operation becomes difficult. For this reason, the strength of the advance assisting spring needs to be set to such a degree as to allow control in the retarding direction at the minimal oil pressure. Moreover, although this valve opening/closing timing control technique is designed to lock the phase to the initial phase at the time of starting the engine, for more reliable engine start, it is desired that the locking to the initial phase be completed before the stopping of the engine.    Patent Document 1: Japanese Patent No. 3211713 (e.g. paragraphs 36-57)    Patent Document 2: Japanese Patent Application “Kokai” No. 2002-227621 (e.g. paragraphs 50-59).